1. Field of the Invention
The invention relates to a device and method of controlling a motorcycle, and more particularly to a device and method for detecting a slip quantity of a motorcycle.
2. Description of Related Art
Various traction control methods for motorcycles are known (JP-A-8-232697, for example). One such traction control method is based on a slip ratio calculated on the basis of rotation speeds of a driving wheel and a driven wheel. The driving force of the motorcycle is controlled in accordance with the calculated slip ratio.
A ratio of circumferential length of tires between front and rear wheels, however, changes in accordance with an inclining angle of the motorcycle (referred to as a “bank angle”, hereinafter). Accordingly, the slip ratio is calculated when the motorcycle is inclined even when the motorcycle does not slip. Further, when the motorcycle has slipped, inclining the motorcycle causes a slip ratio larger or smaller than a slip ratio suitable for an actual slip degree to be calculated. Thus, it is impossible to carry out accurate traction control in accordance with this method.
In view of the above, JP-A-8-232697 proposes a traction control method taking bank angle into consideration. In JP-A-8-232697, a degree of acceleration slip is determined on the basis of a width of a change in rotation number of an engine. Engine output is controlled by delaying an angle of engine ignition timing in accordance with the determined degree of slip. In the traction control method of JP-A-8-232697, the larger the bank angle is, the larger the delay angle of the engine ignition timing is. Moreover, the delay quantity of the engine ignition timing is further increased in banking in accordance with an increase in width of a change in rotation number of the engine.
JP-A-8-232697 describes that engine output is suppressed in banking, which allows slip of a driving wheel in banking to be prevented in accordance with the above method.
The ratio of circumferential length of tires between the front and rear wheels is not only dependent on the bank angle. For example, the ratio of circumferential length also changes when the motorcycle speed changes, and thereby, an expansion ratio of a tire changes. Thus, accurate traction control cannot be performed even with the traction control method of JP-A-8-232697.